I doubt there's much more than a cameo left for Gonnosuke, his fights have probably been traded for fights against Itto Ittosai.
There are basically two things left to show now, before the fight. First is Kojiro becoming a swordsmaster, but that could be done in a couple of chapters, and Musashi getting in touch with the Hosokawa, and having is fight against Kojiro set up. Another half-book and that's it. Keep one book for the final fight. And a couple of chapters for Musashi's later life. That's 2,5 volumes left.
I think it'll look like that... Sad that so much has been left or will be left aside, though.